The present invention relates to a current source circuit formed in a semiconductor integrated circuit (IC) and, in particular, to a bipolar type very small current source circuit, demanding an output current of high accuracy, which is used as a current source of an electronic circuit for application to an automobile, home electric appliance, industrial machine and equipment.
A conventional bipolar type very small current source circuit as shown in FIG. 1 is represented by a power source node 21 supplied with a current source voltage Vcc, a current output node 22 (a current drawing node in this example) to which a load circuit is connected, and ground GND, in which an input current source circuit 23 and first NPN transistor Q1 of a multi-emitter configuration are connected, in a series array, between the current source node 21 and the GND, the input current source circuit 23 supplying an input current Iin and the first NPN transistor Q1 having its collector and base connected to each other, its collector connected to an output of the input current source circuit 23 and its multi-emitter side connected to the GND.
Further, a collector-to-emitter circuit of a second NPN transistor Q3 is connected between the current output node 22 and the GND and has its base connected to the base of the transistor Q1.
In the current source circuit above, the collector current in the transistor Q1 is substantially Iin and if, in this case, those emitter areas A1 and A2 of these transistors Q1 and Q3 are given as a ratio of K (integer):1, then an output current Iout flowing through the transistor Q3 and output node 22 is given as EQU Iout=Iin/K.
It is necessary that, in order to obtain a very small output current Iout in a current attenuation circuit for instance, the emitter area ratio K:1 be made greater. In order to set K=25 for instance, the pattern size of the transistor Q1 becomes considerably greater (size corresponding to 25 transistors) and, correspondingly, the size of the IC chip becomes considerably greater.
A very small current source circuit of FIG. 2 constituting another prior art circuit is represented by a power supply node 11 supplied with a power source voltage Vcc, current output node 12 (current drawing node in this example) connected to a load circuit, and ground GND. Between the power source node 11 and the GND is connected a series circuit of a reference current source circuit 13 supplied with a reference current Iref, a collector-to-emitter circuit of a first NPN transistor Q1 having its collector and base connected to each other, and a collector-to-emitter circuit of a second NPN transistor Q2.
Further a collector-to-emitter circuit of a third NPN transistor Q3 and resistive element R are connected, in a series array, between the power source node 11 and the GND.
The base of the transistor Q1 is connected to that of the transistor Q3 and the base of the transistor Q2 is connected to the emitter of the transistor Q3.
Further, a collector-to-emitter circuit of a fourth NPN transistor Q4 is connected between the current output node 12 and the GND. The base of the transistor Q4 is connected to the collector of the transistor Q2.
In the current source circuit above, the base-to-emitter forward voltage of the transistor Q1 is represented by VBEQ1, base-to-emitter forward voltage of the transistor Q2 by VBEQ2, base-to-emitter forward voltage of the transistor Q3 by VBEQ3 and base-to-emitter forward voltage of the transistor Q4 by VBEQ4 and collector current (output current) of the transistor Q4 by Iout, then a potential Vx on the base of the transistor Q4 is given by: EQU Vx=VBEQ2+VBEQ3-VBEQ1 EQU =VT.multidot.1n{Iref/(.beta..multidot.Is)}+VT.multidot.1n{VBEQ2/(R.multidot ..beta..multidot.Is)}-VT.multidot.1n{Iref/(.beta..multidot.Is)} EQU =VT.multidot.1n[{Iref/(.beta..multidot.Is)}* {VBEQ2/(R.multidot..beta..multidot.Is)}* {.beta..multidot.Is/Iref}] EQU =VT.multidot.1n{VBEQ2/(R.multidot..beta..multidot.Is)} EQU =VT.multidot.1n{Iout/(.beta..multidot.Is)} (1)
Here, VT: thermal voltage
.beta.: current amplification factor
Is: saturation current
From the equation (1) the following equation (2) is found: EQU Iout=VBEQ2/R (2)
That is, the output current Iout is proportional to the reciprocal (1/R) of the resistive value of the resistive element R. In order to obtain a very small output current Iout, it is only necessary to make the resistive value of the resistive element R greater. In this case, however, the pattern size of the resistive element R is made considerably greater and hence the size of the IC chip becomes considerably bulkier.
The conventional current source circuit above has the drawback in that, in order to obtain a very small output current, the pattern size of those elements used becomes considerably greater and hence the size of the IC chip becomes considerably greater.